1. Field
The present invention relates generally to equalization of a received signal, and more specifically to hybrid decision feedback equalization.
2. Background
The transmission of digital information typically employs a modulator that maps digital information into analog waveforms. The mapping is generally performed on blocks of bits contained in the information sequence to be transmitted. The waveforms may differ in amplitude, phase, frequency or a combination thereof. The information is then transmitted as the corresponding waveform. The process of mapping from the digital domain to the analog domain is referred to as modulation.
In a wireless communication system, the modulated signal is transmitted over a radio channel. A receiver then demodulates the received signal to extract the original digital information sequence. At the receiver, the transmitted signal is subject to linear distortions introduced by the channel, as well as external additive noise and interference. The characteristics of the channel are generally time varying and are therefore not known a priori to the receiver. Receivers compensate for the distortion and interference introduced by the channel in a variety of ways. One method of compensating for distortion and reducing interference in the received signal employs an equalizer. Equalization generally encompasses methods used to reduce distortion effects in a communication channel. From the received signal, an equalizer generates estimates of the original digital information.
Current equalization methods are based on assumptions regarding the received signal. Such assumptions are generally not correct over a variety of coding, modulation and transmission scenarios; and, therefore, these equalizers do not perform well under many conditions. Additionally, current equalizers employing decision feedback often suffer from error propagation effects that amplify the effect of isolated decision errors. Additionally, the decision feedback process involves hard decisions regarding each symbol and does not consider the likelihood that a symbol decision is correct.
There is, therefore, a need in the art for an equalization method that reduces linear distortion in a received signal over a variety of operating conditions. Still further, there is a need to reduce error propagation in a decision feedback equalizer. Additionally, there is a need to provide a likelihood measure to the decision feedback process.